It is well known that polycarboxylic acid anhydrides are useful as curing agents for various epoxy resins. Such epoxy-anhydride compositions are useful, e.g., as coating compositions, adhesives, potting compounds and composites. However, since most polycarboxylic acid anhydrides are solid at room temperature, difficulties are encountered in trying to use these solid anhydrides as curing agents for liquid epoxy resins. One might attempt to incorporate the solid anhydride, in powdered form, into the epoxy resin. However, incorporation of a powder into a viscous liquid is very difficult and agglomeration usually occurs with air and moisture entrapment. The general procedure for blending a solid anhydride with a liquid epoxy resin is to heat the anhydride above its melting point and mix it with the resin at this temperature. However, operating at these high temperatures greatly shortens the "pot life" of the mixture so that it is difficult to mold or otherwise utilize large volumes of the epoxy-anhydride mixture before it sets to a hard, unarrangeable mass. Furthermore, the requirement of heating the curing agent and epoxy resin in order to be able to mix them together adds to the trouble and cost of the process.
There has been much investigation in recent years of methods of making room temperature liquid polycarboxylic acid anhydrides from solid anhydrides. It has generally been found that various anhydrides can be melted together to form low melting mixtures or eutectics which may be liquid at room temperature. For example, as taught in U.S. Pat. No. 3,296,148, a mixture of hexahydrophthalic anhydride, M.P. 36.degree. C., and tetrahydrophthalic anhydride, M.P. 101.degree. C., forms a low melting eutectic which is liquid above 23.degree.-24.degree. C. However, such liquids eventually solidify, or at least deposit solids, and consequently, one cannot be certain how long such mixtures will remain liquid, even when stored under anhydrous conditions. Various organic compounds, such as isocyanates, polyether polyols, and those containing an oxirane group, have been proposed as additives to stabilize these liquid anhydride blends. However, inclusion of these various stabilizing materials into the liquid anhydrides may require the use of elevated temperatures, e.g., 80.degree. C. or higher, to prepare the stabilized mixture, require additional excessive costly amounts of stabilizer, and/or adversely affect the properties of the anhydride mixture and the epoxy resin cured therewith.
High melting point anhydrides, such as those having melting points greater than 150.degree. C., however, are not particularly suitable for forming low melting eutectics. These high melting point anhydrides are used whenever epoxy-anhydride products having high glass transition temperatures, i.e., greater than about 150.degree. C., are required. In order to incorporate these high melting point anhydrides into the epoxy resin, they are first heated to their melting point and then incorporated in the epoxy resin. However, as discussed previously, such processing limits the "pot life" of the mixture.
We have now found that a specific combination of anhydrides forms a uniform paste which can be easily dispersed into liquid epoxy resins at room temperature to form a liquid composition that cures uniformly.